Filtering of a receive frequency band

ABSTRACT

The invention relates to the filtering of a subfrequency band out of a receive frequency band, in which a carrier frequency for prefiltering the receive frequency band and an intermediate frequency for demodulating the frequency band filtered out during the prefiltering and for generating a frequency baseband are inserted into the signal propagation path, in which the desired subfrequency band is filtered out of the frequency baseband by a post-filtering and in which the carrier frequency, the intermediate frequency and the post-filtering are matched to one another.

FIELD OF INVENTION

The invention relates to a method and a receiver for receivingtransmitted signals which can be transmitted in various subfrequencybands of a receive frequency band.

BACKGROUND

It is of advantage, especially in radio systems for transmittingtransmitted signals by radio but also in line-connected transmissionsystems, if the transmitted signals can be received in varioussubfrequency bands of a receive frequency band. Individual subfrequencybands differ from one another with regard to their bandwidth and/or withregard to their frequency in the receive frequency band. By changingfrom a first subfrequency band to a second subfrequency band which has agreater bandwidth, transmitted signals with a greater transmission ratecan be transmitted, for example. In a boundary case, the subfrequencyband used is equal to the receive frequency band, i.e. the maximumavailable frequency bandwidth is utilized by the subfrequency band.

It is known to filter out a signal frequency band containing thetransmitted signals by tuning the receive frequency band to a carrierfrequency and by filtering the receive frequency band in a receiver. Thesignal frequency band is then demodulated in a demodulator so that afrequency baseband containing the transmitted signals is available atthe output of the demodulator. The frequency baseband is processedfurther, for example, in that the information contained in it isdigitized by means of an analog/digital converter and conditioned forits intended use as transmitted signals by subsequent fine filteringand/or further processing steps.

Surface acoustic wave filters (SAW filters) are known as filters forfiltering a signal frequency band out of the receive frequency band assubfrequency band. However, SAW filters have the disadvantage of beingrelatively expensive to manufacture or procure.

To filter out subfrequency bands of different bandwidths in aradio-frequency section of a receiver, a plurality of SAW filters and/orSAW filters with different filter bandwidths are used. Each SAW filteror each filter bandwidth, respectively, corresponds to a bandwidth ofone of the subfrequency bands which can be filtered out in the receiver.Because of the plurality of SAW filters or the plurality of filterbandwidths, such a receiver is relatively expensive. Furthermore,additional, relatively expensive switching elements, for example PINdiodes, are needed for switching to a different SAW filter or to adifferent filter bandwidth, respectively, when the subfrequency band ischanged.

From U.S. Pat. No. 5,604,746, a method and receiver for receivingtransmitted signals which can be transmitted in various subfrequencybands of a receive frequency band is known in which a first signalfrequency band containing the transmitted signals is filtered out byadding a carrier frequency to the receive frequency band and byprefiltering the receive frequency band, in which a frequency basebandcontaining the transmitted signals is generated by demodulating thefirst signal frequency band by adding an intermediate frequency to thefirst signal frequency band, and in which at least one second signalfrequency band containing the transmitted signals is filtered out of thefrequency baseband by post-filtering, the carrier frequency and/or theintermediate frequency being matched to one or more filter parameters inthe post-filtering in such a manner that the desired subfrequency bandis available as a second frequency band.

It is the object of the present invention to specify a method and areceiver for receiving transmitted signals which can be transmitted invarious subfrequency bands of a receive frequency band, the applicationor use of which requires little hardware costs.

SUMMARY

In the method according to the invention, a first signal frequency bandcontaining the transmitted signals is filtered out by adding a carrierfrequency to a receive frequency band and by prefiltering of the receivefrequency band. A frequency baseband, the bandwidth of which preferablyessentially corresponds to the bandwidth of the first signal frequencyband and which contains the transmitted signals is generated by addingan intermediate frequency to the first signal frequency band and bydemodulating the first signal frequency band. A second signal frequencyband containing the transmitted signals is then filtered out of thebaseband by post-filtering. The carrier frequency and/or theintermediate frequency are matched to one or more filter parametersduring the post-filtering in such a manner that the desired subfrequencyband is available as second signal frequency band. After that, theinformation contained in the first signal frequency band is digitized.In particular, the demodulation and the post-filtering is then performedas fine filtering on the digitized information. In this process, thepost-filtering is matched to the carrier frequency and/or theintermediate frequency. At the device end, a digital filter is thusprovided in order to filter the transmitted signals out of the digitizedinformation. The digital filter can be driven by the common frequencyand post-filter control of the pre-filter and post-filter.

According to a core concept of the invention, the desired subfrequencyband is filtered out by combined pre- and post-filtering with matchedfilter frequencies or filter parameters during the prefiltering andpost-filtering. An essential advantage of this concept is that duringthe prefiltering, a filter having a fixed invariable filter bandwidthcan be used. This makes it possible to save costs which, on the otherhand, do not occur in the same magnitude during post-filtering since thefiltering of the desired subfrequency band out of the baseband can beimplemented much less expensively.

The receiver according to the invention has a first oscillator forcoupling a carrier frequency into a receiving path of the receivefrequency band. In the receiving path, a prefilter is arranged forfiltering a first signal frequency band containing the transmittedsignals out of the receive frequency band matchd to the carrierfrequency. Furthermore, a second oscillator is provided for coupling anintermediate frequency into a first signal path of the first signalfrequency band. In the first signal path, a demodulator is arranged fordemodulating the first signal frequency bandwith the insertedintermediate frequency and generating a frequency baseband, thebandwidth of which essentially corresponds to the bandwidth of the firstsignal frequency band and which contains the transmitted signals. In abase path of the frequency baseband, a post-filter is arranged forfiltering a second signal frequency band containing the transmittedsignals out of the frequency baseband. A common frequency andpost-filter control of the post-filter and of the first oscillatorand/or the second oscillator is provided for tuning the carrierfrequency and/or the intermediate frequency with one or more filterparameters of the post-filter in such a manner that the desiredsubfrequency band is available as the second signal frequency band. Acommon frequency and post-filter control is not only a central controlbut also a distributed control, a control unit of the post-filtersupplying, for example, information to a control unit of the firstand/or second oscillator and/or conversely. Furthermore, the informationcontained in the first signal frequency band is digitized. Inparticular, the demodulation and post-filtering is then performed asfine filtering on the digitized information. In this process, thepost-filtering is matched to the carrier frequency and/or theintermediate frequency. At the device end, a digital filter is thusprovided in order to filter the transmitted signals out of the digitizedinformation. The digital filter can be driven by the common frequencyand post-filter control of the pre-filter and post-filter.

The post-filter preferably exhibits a low-pass filter or a high-passfilter or a high-pass/low-pass filter combination, the cut-off frequencyof which or cut-off frequencies of which of which are matched to thecarrier frequency and/or the intermediate frequency in such a mannerthat the cut-off frequency or cut-off frequencies separate the desiredsubfrequency band from all neighboring frequency bands which may stillbe present in the frequency baseband. Low-pass filters and/or high-passfilters for filtering the frequency baseband can be inexpensivelyimplemented, for example by means of cascaded RC sections which arecomponents of a single integrated circuit. However, other solutionsknown per se can also be selected, for example operational amplifierswhich have feedback with RC sections.

In particular, the intermediate frequency which determines the zerofrequency value of the frequency baseband is selected in such a mannerthat the zero frequency value is in the center of the desiredsubfrequency band.

When the method according to the invention is carried out, only a singlesubfrequency band, namely the desired subfrequency band, is filtered outin many cases. In a variant, however, a plurality of desiredsubfrequency bands is filtered out.

In particular, a high-pass filter or a low-pass/high-pass filtercombination is used in the post-filtering in such a manner for filteringout the single desired subfrequency band or one of the desiredsubfrequency bands which is not symmetric to the zero frequency of thefrequency baseband. If correspondingly, both a low-pass filter and ahigh-pass filter and/or a high-pass/low-pass filter combination is usedduring the post-filtering, both a desired subfrequency band is filteredout which is symmetric to the zero frequency value and one or moresubfrequency bands which are not symmetric to the zero frequency valueof the frequency baseband are filtered out.

If the desired subfrequency band or one of the desired subfrequencybands is filtered out by high-pass filtering of the frequency basebandor a combination of high-pass and low-pass filtering of the frequencybaseband, the desired subfrequency band being either in the positive orin the negative frequency range of the frequency baseband, thesubfrequency band filtered out is preferably digitized after thefiltering-out and transposed by the digital conversion into a frequencyrange which contains the zero frequency value. In particular, thedesired subfrequency band thus obtained is then symmetric to the zerofrequency value.

In a preferred further development, the carrier frequency for theprefiltering is set in such a manner that one or more neighboringfrequency bands of the desired subfrequency band are already split offduring the prefiltering. In particular, the combination of the presetfilter frequency bandwidth used during the prefiltering and thearbitrarily adjustable carrier frequency which is coupled into thereceiving path acts like a freely adjustable band-pass filter. In thismanner, either all neighboring frequency bands above or all neighboringfrequency bands below the desired subfrequency band can already be splitoff by the prefiltering. This is of advantage, in particular, whenneighboring frequency bands having a greater receive field strength thanthe desired subfrequency band are received. However, adjusting thelimits of the prefilter frequency range, i.e. adjusting the band-passcut-off frequencies, is preferably also matched to the choice ofintermediate frequency and the choice of the filter parameter orparameters of the post-filter.

It is known per se to provide on the signal path between the prefilterand the demodulator an amplifier arrangement which optimally matches thelevel of the frequency band filtered out during the prefiltering to thedemodulator. If then there is in the first signal frequency band aplurality of subfrequency bands, one of which is the desiredsubfrequency band, and if the desired subfrequency band has a lowerfield strength or a lower level than one or more of the neighboringfrequency bands, it is advantageous if the second signal frequency bandis amplified after the post-filtering has been performed at leastpartially. In this manner, the level of the desired subfrequency bandwhich is low due to the matching to the demodulator is raised,preferably to a level value which is matched to any subsequentprocessing stages. At the device end, a second signal band amplifier foramplifying the second signal frequency band is therefore arranged in asecond signal path of the second signal frequency band following thepost-filter or, respectively, following the first part of the postfilter in the direction of signal propagation in a further development.

In a further development, the second signal path exhibits a bypass forunamplified forwarding of the second signal frequency band, which isconnected in parallel with the second signal band amplifier.

The second signal band amplifier is composed of a plurality ofindividual amplifiers, in particular, for example of two individualamplifiers having a low-noise input amplifier stage.

An embodiment of the receiver according to the invention is especiallypreferred in which the second signal band amplifier which may be presentand at least a part of the post-filter are arranged in a commonintegrated circuit.

It is also especially preferred if the demodulator and at least part ofthe post-filter are arranged in a common integrated circuit. Thecorrespondingly high degree of integration saves production costs andspace.

At the device end, the analog/digital converter is then arranged behindthe prefilter and in front of the digital demodulator in the directionof signal propagation.

A digital demodulator, especially a digital I/Q demodulator, performs adigital down-conversion of the first signal frequency band, for examplefrom frequency ranges around 10 MHz into the frequency baseband.

The method according to the invention is not restricted to receivefrequency bands, the subfrequency bands of which are in non-overlappingfrequency ranges. Instead, the method can also be used in the case ofsubfrequency bands which overlap one another. For example, subfrequencybands according to the OFDM (Orthogonal Frequency Division Multiplex)modulation method overlap. However, the field strength of the nextsubfrequency band adjoining a certain subfrequency band is approximatelyzero at the frequency value at which the certain subfrequency band hasits maximum field strength. In this sense, the individual subfrequencybands are orthogonal subfrequency bands. When the receive frequency bandis sampled, the certain subfrequency band can then be sampled in thearea of its peak so that, at the most only small signal components andin the ideal case no signal components of adjacent subfrequency bandsare also detected. Since there are no distinct frequency boundaries ofthe individual subfrequency bands in the case of subfrequency bandswhich overlap, the cut-off frequencies of the subfrequency bands arereplaced by meaningful separating frequencies at which separation takesplace between a filtered-out frequency range and frequency ranges whichare cut off during the filtering of the receive frequency band and thefurther frequency bands filtered out of the receive frequency band.Separating frequencies must be selected in such a manner that thefrequency ranges filtered out contain the desired subfrequency bands andthe desired subfrequency band in useable form, i.e. the informationcontained therein can be used and the signal components of the frequencyranges cut off do not impede the evaluation and/or render it impossible.

The present invention will now be explained in greater detail withreference to exemplary embodiments. However, it is not restricted tothese exemplary embodiments. In the description which follows, referenceis made to the attached drawing in the individual figures of which:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an especially preferred embodiment of a receiver accordingto the invention, and

FIG. 2 shows the receive field strengths of adjacent subfrequency bandsof a receive frequency band.

FIG. 3 shows the receive frequency band of FIG. 2 after a carrierfrequency has been added or inserted, respectively,

FIG. 4 shows the first signal frequency band filtered out of the receivefrequency band of FIG. 3,

FIG. 5 shows the frequency baseband generated from the first signalfrequency band of FIG. 4, and

FIG. 6 shows the desired subfrequency band filtered out of the frequencybaseband of FIG. 5.

DETAILED DESCRIPTION

FIG. 1 shows a receiver according to the invention for receiving radiosignals which are transmitted in a future radio system, the UMTS(Universal Mobile Telecommunication System). For certain operatingmodes, for example the uncoordinated operation of a multiplicity ofmobile telephones, frequency bands of limited bandwidths are available.In the example shown, the receive frequency band has a frequencybandwidth of 4.096 MHz. The carrier frequencies for the radiotransmission of the transmitted signals are in the range of 2 GHz.

The receiver shown in FIG. 1 has at its input first an input amplifier 1and an input filter 2 in the direction of signal propagation as is knownfrom the prior art. The input filter 2 is used for coarse filtering outof the frequency range used in the UMTS. In the direction of signalpropagation, the input filter 2 is followed by carrier frequencyinsertion 3 at which the respective carrier frequency generated by acarrier frequency oscillator 15 is inserted into the signal propagationpath.

In the direction of signal propagation, the carrier frequency insertion3 is first followed by a surface acoustic wave (SAW) filter 4, twoseries-connected LNAs (Low Noise Amplifiers) 5, 6 and an I/Q(in-phase/quadrature) demodulator 7. The I/Q demodulator 7 is providedwith an intermediate frequency which specifies the zero frequency valueof the frequency baseband which is generated by the I/Q demodulator 7 bydemodulating a frequency band present at its input by anintermediate-frequency oscillator 16.

At the output end of the I/Q demodulators 7, a section of the signalpropagation path follows in which first a variable low-pass filter 8 isarranged. This is followed by a series circuit of two further LNAs 9,10, a bypass 14 which allows output signals of the low-pass filter 8 tobe forwarded unamplified to the analog/digital (A/D) converter 11following it in the direction of signal propagation being connected inparallel to the LNAs 9, 10. The A/D converter 11 is followed by adigital filter 12 and then by a Rake combiner 13 for combiningindividual components of the receive signal which have been received bythe receiver with time offset, for example due to multi-pathpropagation.

The LNAs 5, 6 and the LNAs 9, 10 are driven by an automatic amplifiercontrol 20. The amplifier control 20 firstly drives the LNAs 5, 6 insuch a manner that the output level of the LNA 6 is optimally matched tothe I/Q demodulator 7. As a result, a sufficiently high output level isachieved at the output end of the I/Q demodulator 7 and, on the otherhand, overdriving of the I/Q demodulator 7 is prevented. Furthermore,the LNAs 9, 10 are driven by the automatic amplifier control 20 in sucha manner that the optimum input level is present at the A/D converter11. If the output level of the low-pass filter 8 is sufficiently high,the signal is forwarded directly to the A/D converter 11 via the bypass14 without amplification. For this purpose, switching means, not shownin greater detail, are provided which are also driven by the automaticamplifier control 20.

A combined frequency and post-filter control drives the carrierfrequency oscillator 15, the intermediate-frequency oscillator 16, thelow-pass filter 8 and the digital filter 12, in such a manner thattransmitted signals of a desired subfrequency band are present at theoutput of the digital filter 12. For this purpose, a device, not shown,of the frequency and post-filter control 18 provides both the frequencyand the bandwidths of the desired subfrequency band. The signalprocessing controlled by the frequency and post-filter control 18 willbe discussed in greater detail below with reference to an exemplaryembodiment.

The low-pass filter 8 consists of cascaded RC sections which arearranged in a common integrated circuit with the I/Q demodulator 7, theLNAs 9, 10 and the bypass 14 on one chip. The entire integrated circuitcan thus be produced inexpensively in large numbers with littleadditional cost for the RC sections. The entire filter combinationconsisting of the SAW filter 4, the low-pass filter 8 and the digitalfilter 12 can thus be produced less expensively than in the prior art.

The receiver shown in FIG. 1 can be used for filtering out subfrequencybands having channel bandwidths of 0.256 MHZ, 0.512 MHz, 1.024 MHz,2.048 MHz and 4.096 MHz, a subfrequency band having a bandwidth of 4.096MHz corresponding to the entire receive frequency band. Since thebandwidth of all possible subfrequency bands can be achieved by dividingby means of one integral number on the receive frequency bandwidth, thehardware complexity in the digital area of the receiver can be keptdown. In particular, the operating mode of the digital area can besimply adapted to a lower subfrequency bandwidth by reducing the clockrate to the corresponding fraction.

Referring to FIG. 2 to FIG. 6, the filtering-out of a subfrequency bandin the receiver shown in FIG. 1 is now explained by way of example. FIG.2 shows a receive frequency band of frequency bandwidth W_(R) with atotal of four subfrequency bands in each case of frequency bandwidthW_(Sub), and other frequency bands in which no transmission oftransmitted signals is to be expected even if the radio channel ischanged. The subfrequency bands are designated by letters a to d. In theexemplary embodiment, subfrequency band c is to be filtered out.Subfrequency band c corresponds to a transmission channel via which, forexample, voice data are transmitted from a base station to a mobiletelephone. The diagram according to FIG. 2 represents a snapshot. Thefrequency and the frequency bandwidth W_(sub) of the desiredsubfrequency band can change by a change in the operating situation inthe entire transmission system or in part-areas of the transmissionsystem, the UMTS in the example. In particular, subfrequency bands withdifferent frequency bandwidths W_(sub) can also be present at anotherpoint in time within the limits of the subfrequency bands a to d in thereceive frequency band.

For the sake of simplicity, the text which follows is based on theassumption that the frequency bandwidth W_(R) of the receive frequencyband is 4 MHz, that the frequency bandwidths W_(sub) of the individualsubfrequency bands are in each case 1 MHz and that the receive frequencyband is transmitted to a receiver by means of a carrier frequency of1.900 GHz. During the transmission to the receiver, the receivefrequency band extends within the frequency range of 2.000 GHz to 2.004GHz. The desired subfrequency band is within the frequency range of2.002 GHz to 2.003 GHz.

The field strengths of the individual subfrequency bands a to d areshown in FIG. 2. The diagram shows the unfavorable situation, from thepoint of view of processing, that the field strength of the desiredsubfrequency band c is less than the field strengths of the two closestadjacent subfrequency bands b, d. During the processing of the receivesignal, the carrier frequency is inserted into the signal propagationpath at the carrier frequency insertion point 3. It is normallyattempted to insert precisely the carrier frequency known to thereceiver as the carrier frequency which was used in transmitting thereceive signal. In the present case, however, a receive carrierfrequency is inserted which is dematchd with respect to the transmitcarrier frequency of 1.900 GHz. For this purpose, the frequency andpost-filter control 18 drives the carrier frequency oscillator 15 insuch a manner that it generates a receive carrier frequency of 1.902GHz. This receive carrier frequency is inserted at the carrier frequencyinsertion point 3.

Due to the insertion of the receive carrier frequency and subtraction, areceive frequency band with reduced frequency is formed in which thedesired subfrequency band c is within the frequency range of 100 MHz to101 MHz.

The SAW filter 4 is set unalterably to a frequency bandwidth whichcorresponds to the frequency bandwidth of the receive frequency band. Inthe example, this is a frequency bandwidth of 4 MHz. The SAW filter 4filters a frequency band out of the frequency band present at its input,the lower boundary value of which is equal to 100 MHz. In the presentexample, the SAW filter 4 thus filters out a first signal frequency bandwhich is within the frequency range of between 100 MHz and 104 MHz. Thesubfrequency bands a, b are thus already no longer present in the firstsignal frequency band (FIG. 4). The figure also shows the envelope curveof the SAW filter 4.

The receive carrier frequency was selected in such a manner that theimmediately adjacent subfrequency band b with the greater field strengthwas split off from the desired subfrequency band c. If the fieldstrength of the subfrequency band d had been greater than the fieldstrength of the subfrequency band b, the receive carrier frequency wouldhave been set to the value of 1.899 GHz so that subfrequency band bwould have been split off.

The first signal frequency band is amplified by the automatic amplifiercontrol in such a manner that the field strength of subfrequency band dcorresponds to the optimum level of the I/Q demodulator 7 at the inputof the I/Q demodulator. To demodulate the receive signal or the firstsignal frequency band, respectively, the frequency and post-filtercontrol 18 drives the intermediate frequency oscillator 16 in such amanner that an intermediate frequency of 100.5 MHz is provided to theI/Q demodulator 7. In general, the intermediate frequency must beselected in the exemplary embodiment in such a manner that the frequencyvalue in the center of the desired subfrequency band is equal to theintermediate frequency. This is because, during the demodulation, afrequency baseband is generated which extends around the zero frequencyvalue and the frequency bandwidth of which is equal to the frequencybandwidth of the first signal frequency band due to the prefiltering.

In the present exemplary embodiment, a frequency baseband is generatedin which subfrequency bands which are still present and adjacentfrequency ranges having level values of greater than zero are within thefrequency range of between −0.5 MHz and +3.5 MHz (see FIG. 5). Theboundary position of the desired subfrequency band is then utilized inthe next processing step. In this step, a frequency range, the value ofwhich is below the cut-off frequency set in the low-pass filter 8, isfiltered out of the frequency baseband in the low-pass filter 8. In thepresent case, the cut-off frequency of the low-pass filter 8 is set to avalue of 0.5 MHz in that the frequency and post-filter control 18appropriately drives the low-pass filter 8. As a result, only thedesired subfrequency band c is present as rest of the frequency basebandat the output end (FIG. 6). The output signal is then supplied to theLNAs 9, 10 and its level is amplified to the optimum value for the A/Dconverter 11.

The digitized information obtained from the desired subfrequency band cis subjected to fine filtering in the w digital filter 12, correctionsbeing performed on the digitized information in accordance with thebandwidth of the subfrequency band c in order to obtain the desireddigital transmit signal. The common frequency and post-filter control 18appropriately drives the digital filter 12 for this purpose. Thedigitized transmit signals are supplied to the Rake combiner 13.

In a further development of the receiver described in the exemplaryembodiment, a band-pass filter or a high-pass filter is also used inaddition to a low-pass filter in the post-filtering in order to filterout the subfrequency bands most closely adjacent to the desiredsubfrequency band or the subfrequency band most closely adjacent to thedesired subfrequency band. From the signals or field strengths,respectively, of these subfrequency bands most closely adjacent or thesubfrequency band most closely adjacent, a total power can bedetermined. The total power determined makes it possible to optimize theposition of the prefilter implemented by the SAW filter 4 for thereception of the desired subfrequency band.

1. A method for receiving signals transmitted in frequency bands of areceive frequency band of a cellular mobile communication system, themethod comprising: obtaining a first signal frequency band containingthe signals by adding a carrier frequency to the receive frequency bandand by pre-filtering the receive frequency band; generating a frequencybaseband containing the signals by adding an intermediate frequency tothe first signal frequency band and by demodulating the first signalfrequency band; performing post-filtering on the frequency baseband toobtain a second signal frequency band containing the signals, whereinpost-filtering is performed by a post-filter having a cut-off frequencythat is variable and that is matched to one or more of the carrierfrequency and the intermediate frequency in order to separate the secondsignal frequency band from a neighboring frequency band; digitizinginformation in the second frequency band to obtain digitizedinformation; fine-filtering the digitized information to obtain thesignals in digital form; and amplifying signals of the second signalfrequency band or bypassing amplifying the signals of the second signalfrequency band based on a post-filter output level of the signals. 2.The method of claim 1, wherein the post-filter comprises one or more ofa low-pass filter, a high-pass filter, and a high-pass/low-pass filtercombination.
 3. The method of claim 1, wherein amplifying comprisesamplifying the second signal frequency band after post-filtering hasbeen at least partially performed.
 4. The method of claim 1, furthercomprising: setting the carrier frequency to split off a neighboringfrequency band of the second frequency band during prefiltering.
 5. Themethod of claim 1, further comprising: digitizing the first signalfrequency band, wherein the frequency baseband is generated throughdigital demodulation.
 6. The method of claim 1, further comprising:performing one of a high-pass filtering and a combination of high-passand low-pass filtering to filter out at least one subfrequency band in arange of the frequency baseband; digitizing the at least onesubfrequency band to produce a digitized subfrequency band; andconverting the digitized subfrequency band into a frequency range whichcontains a zero frequency value.
 7. A receiver for receiving signalstransmitted in frequency bands of a receive frequency band of a cellularmobile communication system, comprising: a first oscillator to insert acarrier frequency into a receive path of the receive frequency band; aprefilter to filter a first frequency band containing the signals out ofthe receive frequency band with the carrier frequency; a secondoscillator to insert an intermediate frequency into a first signal pathof the first frequency band; a demodulator to demodulate the firstfrequency band with the intermediate frequency to generate a frequencybaseband containing the signals; a post-filter to obtain a second signalfrequency band containing the signals from the frequency baseband,wherein the post-filter has a cut-off frequency that is variable, andwherein post-filter obtains the second signal frequency band by matchingthe cut-off frequency to one or more of the carrier frequency and theintermediate frequency in order to separate the second signal frequencyband from a neighboring frequency band; a second signal band amplifierto amplify a second frequency band; and a bypass connected in parallelwith the second signal band amplifier for forwarding a non-amplifiedversion of the second frequency band; wherein the bypass is used basedon a post-filter output level of the signals.
 8. The receiver of claim7, wherein the post-filter includes one of a low-pass filter, ahigh-pass filter and a high-pass/low-pass filter combination, thecut-off frequency separating the second frequency band from neighboringfrequency bands.
 9. The receiver of claim 7, wherein: the demodulatorand at least a part of the post-filter are arranged in a commonintegrated circuit.
 10. The receiver of claim 7, further comprising: ananalog/digital converter.
 11. The receiver of claim 7, wherein thepost-filter includes: a common frequency and post-filter control tomatch one of the carrier frequency and the intermediate frequency to thecut-off frequency; an analog/digital converter to digitize informationin the second frequency band; and a digital filter to filter the signalsout of the digitized information.
 12. The receiver of claim 11, wherein:the second signal band amplifier and at least a part of the post-filterare arranged in a common integrated circuit.